SBOA602 November   2024 OPA593

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Current Booster, Push-Pull Topology Output Characteristics
    1. 2.1 Open-Loop Output Impedance
    2. 2.2 Minimizing Zero Crossover Distortion
  6. 3Various Current Booster Configurations
    1. 3.1 Complementary MOSFET versus BJT Current Booster Comparisons
  7. 4Stabilizing a Design for Power Amplifier Driving 1μF Capacitive Load (CL)
    1. 4.1 Op-Amp Driving Resistive Load
    2. 4.2 Op-Amp Driving Capacitive Load and Challenges
    3. 4.3 Open-Loop AC Stability Analysis - Compensating CL Effects Using DFC
    4. 4.4 Closed-Loop Stability Response - Small Signal Step Transient Analysis
    5. 4.5 Effects of Riso on Frequency Response in Dual Feedback Compensation
    6. 4.6 Summary of the DFC Technique
  8. 5Stabilizing the OPA593 and Darlington Current Booster for 1μF Capacitive Load
    1. 5.1 Open-Loop AC Stability Analysis - Composite Op-Amp Driving 1μF CL
    2. 5.2 Closed-Loop Stability Response - Composite Op-Amp's Step Transient Analysis
  9. 6Composite Amplifier's Effective BW and Step Time Response
  10. 7Test Bench Validation
  11. 8Summary
  12. 9References

6 Composite Amplifier's Effective BW and Step Time Response

Note the importance to clarify the concepts of op-amp bandwidth and step timing response in ATE applications, as engineers often confuse these terms.

An op-amp's effective bandwidth is derived from AC small-signal analysis in a linear model, which describes how quickly the amplifier can respond to small-signal changes at both the input and output while maintaining a relatively constant gain. This model defines the frequency range in which the amplifier can perform effectively in a specific closed-loop configuration. In a closed-loop feedback system, the op-amp bandwidth is a first-order approximation based on the gain-bandwidth product.

In contrast, an op-amp's step response or timing requirements are associated with the slew rate and the characteristics of the input signal, which are typically evaluated using the large-signal model. For ATE applications, step timing responses are generally very rapid, typically on the order of 10µs from applying the input signal to stabilizing the output setpoint, which are critical design parameters.

When output stages are loaded with capacitive or inductive components, longer time constants are introduced due to these load parameters. Merely increasing the op-amp bandwidth does not resolve the time delay or significantly enhance the step response.

To improve the step response time in ATE applications, output driving stages must provide a higher current rate over a short duration. For larger capacitive loads, achieving a fast output voltage setpoint requires rapid feedback control and a high rate of current change to drive the capacitive load, represented as C(di/dt). In the OPA593 + current booster configuration, the large signal step response time is determined by the slew rate of the OPA593 and the rate of current change in the booster driver (for example, di/dt). Overall performance is also influenced by the compensated op-amp, output voltage swing, settling time, and transient overshoot or undershoot behaviors. Thus, a trade-off must be made between minimizing output current spikes and optimizing step response time in ATE applications.